An Antenna Coupled Cold-Electron Bolometer for High Performance Cosmology Instruments

Artikel i vetenskaplig tidskrift, 2007

The newly emerging CMB polarization experiments (eg CLOVER, EBEX) employ detectors comprising transition-edge sensors (TES). The detectors will operate at temperatures of approximately 100 mK and will be read out by time division or frequency multiplexed SQUID amplifiers. Although detectors are expected to deliver impressive sensitivity, future space B-mode experiments (eg B-Pol) can benefit greatly from an increase in sensitivity, much higher saturation power, and flexibility in their realization on planar substrates. In this paper we describe a Cold-Electron Bolometer (CEB), which is a serious candidate for the next space cosmology missions. We analyze the suitability of various devices for the 70 GHz channel of the proposed B-Pol polarimeter. The detector may also be of interest to ground-based experiment as a result of the simplicity of its integration to planar circuit technology. The Capacitively Coupled CEB is a planar antenna-coupled superconducting detector with high sensitivity and high dynamic range. The CEB can meet noise requirements with both SQUID and JFET readouts. The SQUID readout can be used the same as for TES bolometers with typical SQUID sensitivity of 0.5 pA/Hz1/2. An attractive realisation of the detector at millimetre wavelengths is to fabricate the CEB directly connected to the antenna on a planar substrate. The proper matching can be achieved by fabrication of an absorber strip of resistance equal to the wave impedance of the antenna. Three variants of the CEB concept have been considered. The optimum realization of a CEB with SIN and SN tunnel junctions gives noise less than photon noise with SQUID readout. Estimations of the CEB noise with a JFET readout (at 300 K and 4.2 K) has shown an opportunity to realize background-limited performance for realistic power loading. Matching to a JFET is best obtained by using the SCEB (with weak superconducting absorber), and choosing a voltage bias in the flat region of the IV curve with very high dynamic resistance. This configuration can gives photon noise limited performance with JFET readout over a wide range of optical loading levels. Another possibility for matching to a JFET is a current-biased series array of CEBs with normal metal absorbers, connected in parallel for HF signal. The antenna-coupled CEB bolometer is easy to couple to a wide range of planar antennae systems, either on bulk or membrane substrates. Applicability of the CEB to B-Pol and similar space missions looks very promising for all of the frequency bands and with both JFET and SQUID readout schemes.